This design is copyrighted and remains the property of Kevin Kennedy/KTA
and may be used for noncommercial uses only. For commercial use of any of
the circuits presented here, please contact me (kkennedy@positive-feedback.com)
for licensing details.

Our readers should note that all PFO DIY projects are
presented for the edification of our readership, and are published without
any representation as to suitability, nor any guarantee or warranty
whatsoever from either Kevin Kennedy/KTA or PFO. DIY'ers are reminded
that all DIY projects assume a sufficient degree of skill/expertise on the
part of the one doing the work, and that damage, accidents or loss may
occur. Audio gear assembly/repair also may expose a DIY'er to potentially
lethal voltages - PROCEED WITH ALL DUE CAUTION, AND AT YOUR OWN RISK.
Caveat lector!

Introduction

I originally built this pre-amplifier about 4 years ago when I could no
longer contain my rampant curiosity regarding the possible performance
advantages of Directly Heated Triodes (DHT's) used for this purpose. My
original intention was to build something simple and inexpensive to allow me
to investigate the possibilities. Since that time the design has evolved
slightly and a few minor performance issues have been resolved. The current
design is flexible, performs well, and has been very reliable—and perhaps
most importantly, I like what I hear. (Insert a grin here.)

Interestingly many early DHT's offer excellent linearity without the
application of any negative feedback. This characteristic alone would seem
to make them ideal for pre-amplifier applications, but in fact there are
many practical issues that have discouraged their use.

Most early DHT tubes
like the 01 and 26 exhibit low mu, operate at substantial plate currents,
and have a plate resistance high enough to make their use in simple single
stage designs questionable. Some types require weird filament voltages
favored for battery operation, and due to their filamentary cathodes AC
power is not an option if an acceptable signal to noise ratio is a desired
outcome. (Read, "lots of hum.")

I chose the 26 for the
simple reason that I had a good selection of these DHT's on hand, and a
dearth of the 01. The 1.5V filament supply issue did not faze me as I
regarded it as a worthy challenge to be overcome in as simple a manner as
possible.

The 26 and 01 are very
similar except for the filament voltage, current, and the maximum plate
voltage rating. From an audio performance perspective either of these types
should provide exemplary performance.

I initially considered
an SRPP design using dissimilar triodes, or a cathode follower topology;
however, I felt that these were less than optimum solutions, as the
additional tubes would impart their own specific sonic signatures and not
allow me to accurately assess the virtues of direct heated triodes in small
signal applications.

Transformer coupling
seemed like the solution to at least one of the problems at hand, so to that
end a vintage pair of UTC HA-133 plate to line transformers were procured.

This transformer was
commonly employed in Ampex recording studio equipment, where excellent
performance was imperative. The transformer is rated as having a response
flat from 30Hz - 20kHz -1dB 1,
a maximum rated plate current of 8mA, a primary impedance of 15KΩ,
and several possible secondary impedances including 600Ω,
which is commonly employed in professional equipment. A permalloy core,
split primaries and secondaries as well as relatively good shielding are
other features of this device.2

Obviously there is a
certain amount of fascination surrounding the application of transformers
and DHT's in a line stage to which I was not immune. These primitive designs
have an excellent reputation for detail, tonal accuracy, and sheer
musicality, attributes which finally overcame my concerns about possible
technical obstacles.

The design featured here
is very customizable, and can be built to suit a wide range of tastes and
budgets.

Several power supply
design options will be discussed, however based on my experience I do strongly recommend that the power supply be built on a separate chassis
to avoid hum and noise from the supply getting into the audio circuitry.

Bear in mind that this
line stage actually has very little overall gain due to the use of
transformer coupling. Gain in my prototype is less than 4dB total and will
be highly dependant on your choice of plate to line transformer. You may
prefer to think of this design as a cross between a passive controller and a
very low gain power amplifier.

The Sound

The pre-amplifier has a definite sonic character, tending to the very
slightly warm, mellow end of the spectrum. It is extremely holographic in
its imaging characteristics, and has excellent extension into the deep bass
and extreme highs. The midrange is—err—"liquid" (nothing else quite
describes it. Insert another grin here!) Transients are cleanly and clearly
delineated without calling attention to them. The tonal balance is natural.
It would be synergistic with a neutral to a somewhat analytical sounding SE
amplifier, where its character might tend to balance out the overall sound.
(For a suitable SE amplifier design, see my website at
www.kta-hifi.net.) Obviously the transformers are playing a role in
this, and it is quite likely that the character of the overall sound is more
dependent on transformer quality than anything else.

I have noted that tube rolling does produce subtle but audible changes in
the character of the pre-amplifier, but the differences are not extreme, and
any good 26 will do the job. I have tried Radiotron UX-226's, CeCo Globe
26's, (my favorite) Philco's and several others—all with pleasing results.
The installation of stepped attenuators recently resulted in a significant
improvement in overall resolution, and I recommend them.

Overall I would say the sound is quite compelling, the signal path is simple
enough that none of the music gets lost along the way. I also recently added
switches to allow me to change the absolute polarity of the outputs, and
there is material where this addition has been clearly beneficial—unprocessed vocals, choral works and percussion instruments.

Disclaimer

Hazardous and possibly even lethal voltages are present in this design.
No responsibility will be taken for the ability or inability to use this
information safely. This article is intended for those with some
construction experience and knowledge of what constitutes safe construction
and operating practices.

Design Considerations

Being originally
somewhat budget-minded orientation on this project, I choose to use as many
junk box components as I could muster. A surplus encapsulated supply was
used to provide both bias and filament power. Any supply that can provide at
least 2 - 3A @ 5.0V and -12V or -15V @ 100mA will serve nicely. For those
who are more ambitious or just don't have the requisite supply in their junk
box, several other options will be mentioned below.

The sockets are
inexpensive Russian made ceramic jobbies with gold plated contacts and have
not caused any problems in 4 years. There are some wonderful NOS, not to
mention new sockets available, for those inclined to spend the money.

The 26 filaments are
operated in series using a regulated 5V supply with dropping resistors to
reduce the voltage to 3V for the filaments... Despite the common filament
connection cross-talk between channels has not been a problem.

I will detail several
filament supply options that do not require the use of surplus power
supplies, as well as what is required to use the 01, and its variants.

While cathode bias is
possible with this design, I felt that sharing a common bias resistor and
capacitor was a design compromise I did not want to make. Early DHT types
are actually well suited to fixed bias operation.

Some will choose to bias
their tubes with 9V batteries which will provide excellent service, and
approximately the right bias voltage; however they do need to be changed
periodically as their internal impedance increases with age. I felt that I
wanted a "use and forget" sort of design not requiring periodic maintenance…
(Hopefully, anyway…)

Batteries do have a
major advantage in that they are entirely free of noise and ripple; however
I prefer a well filtered and regulated bias supply which allows a wide bias
adjustment range to optimize performance.

A fully regulated plate
supply is featured in this design to assure consistent performance and low
noise. The basic regulator design is quite simple and uses readily available
parts. A carefully selected neon lamp can be substituted for the zener
reference; however, feedback resistors will need to be adjusted in value to
compensate for the change, and it is left to the accomplished reader to
determine the correct values for the neon in hand.

Most pre-amplifier
designs use some minimum level of local or global feedback, but this design
does not and is only practical due to the inherent linearity of the tube
chosen.

Performance is excellent
in every sense; however, certain precautions are required in the layout and
construction of this design as both the transformers and the tubes are
susceptible to electrical noise pick up.

Technical Details

The pre-amplifier
topology is a zero feedback common cathode direct heated triode stage
transformer coupled to the load. It is essentially a low gain power
amplifier with a voltage gain of under 4dB. Overall voltage gain is
determined by the mu of the triode which is ~8 for both the 01 and the 26,
the winding ratio of the plate to line transformer chosen, and the load
impedance of the driven power amplifier. The output polarity of each channel
is switchable in order to address absolute polarity issues which are
sometimes audible in my system. They may be included or omitted as the
builder desires.

Bias the 26 to
approximately 6 or 7mA which is roughly equivalent to –9V grid bias. Measure
the DCR of the output transformer and then set the bias to drop the
appropriate voltage across the transformer primary where I=E/R, where I is
the desired bias current, E is the voltage measured across the primary and R
is the measured DCR.

See the table below for
the characteristics of the DHT of your choice:

Tube Type

Filament

Plate
Resistance

MU

Bias

Ip

26

1.5V @
1.05A

8000Ω

8

-9.0V

7mA

01

5.0V @
0.25A

8000Ω

8.5

-9.0V

3mA

Transformers having a
primary impedance of 10KΩ
to 20KΩ
may be employed with good results; however the higher the primary impedance
the less the available gain.

Any transformer chosen
should have a primary inductance in excess of 200H at the desired operating
current if good low frequency response is to be maintained. Note also that
the gain will be influenced by the load impedance reflected back to the
primary of the transformer. In most instances these designs are intended to
drive fairly high input impedances; note that gain decreases and distortion
increases significantly as the load impedance approaches 600Ω,
and 10K and higher load impedances are recommended for this reason.

The table below provides
estimates of voltage gain, and source impedance for a variety of transformer
primary impedances and a secondary impedance of 600Ω.
The effects of DCR are neglected and will reduce gain slightly, and increase
output impedance as well. Note that due to the high effective load impedance
appearing at the tube's plate the available gain will very closely approach mu, provided that the load impedance reflected from the secondary is
~infinite. This table assumes an infinite load impedance—in practice,
anything over 100KΩ
will result in performance very similar to the values noted. Measurement of
the prototype indicate it agrees closely with the prediction for 15KΩ
primary impedance.

Primary
Impedance

Voltage Gain

Output Impedance

10KΩ

+5.8dB

480Ω

15KΩ

+4.0dB

320Ω

20KΩ

+2.8dB

240Ω

Power
Supplies

The power supplies must
all exhibit extremely low noise levels, with the filament and bias supplies
being the most critical.

Plate
Supply

The plate supply
utilizes a power transformer providing a center tapped secondary of 600VCT
minimum to 700VCT at 50mA. Rectification is provided by a GZ32 or GZ34/5AR4
which is overkill for the amount of current required, but provides long warm
up delays to prevent possible cathode stripping in the pass element and most
importantly in the DHT's themselves. It's also readily available and
inexpensive—a consideration. The plate supply shown is of relatively
conventional practice (for me LOL) with a few minor adjustments to allow
operation at low output voltages. The 6BQ5 series pass tube is operated as a
pentode in order to maximize RP and thereby present a high series impedance
to noise on the raw supply. Note that as shown in the schematic, it is
crucially important to bypass the screen grid of V4 at the socket in order
to prevent VHF oscillations. If the power supply is built in a separate
chassis, as I recommend, there should be only one chassis grounding point
with all ground connections brought back to this point. I also recommend
that the center tap of the power transformer be brought directly to the
negative side of the filter caps, and from there go to the star ground—this keeps cap charging currents out of the ground wiring. The error
amplifier is a paralleled 12AX7A and the reference is a bootstrapped 51V zener. The dc closed loop gain is about 10dB which provides an output of
150V. The entire 34dB of open loop gain is available for reducing AC output
impedance and for reducing output noise. Typical noise levels on the output
with good layout should be ≤5mVpp.

Choosing your DHT…

By this point you will
need to decide which DHT you wish to use as the filament supply
configuration will differ significantly depending on the choice you make.
The brick option will allow use of either tube type, but is a less than
ideal solution for use with the 01 tube if there is any significant amount
of noise present on the supply output.

The
"Brick" Option

Several options exist
for the filament regulator circuitry; the simplest is to utilize a surplus
brick type power supply.3
Which is an excellent approach for use with the 26 tube. These usually come
with a 5V supply at several amps, and a ±12V
or ±15V
supply at a few hundred mA to operate op-amps. If you wish to use one of
these supplies to provide bias voltage it is essential that the 5V supply be
completely independent of the other supplies with a separate & independent
ground return. The 26 filaments are operated in series using a regulated 5V
supply with dropping resistors to reduce the voltage to 3V to operate the
filaments. The supply must float, as only the junction between the two tube
filaments is grounded. Some additional filtering is provided by a large
electrolytic capacitor connected across the filaments.

In the case that a brick
supply is used, bias may or may not be derived from one of the other
outputs. An independent bias supply or batteries may be used if desired.
Ripple levels on this supply must be very low, and 0.5mV or less is
recommended to prevent audible buzz.

1. I measured a response that was flat to better than
± 0.5dB from 20Hz – 20kHz
which indicates that this component was conservatively specified by
UTC.

2. Magnetic coupling is always an issue with low level
audio transformers and I recommend mounting them as far away as
possible from power transformers and other AC wiring.

3. These supplies are made
by Acopian, Kepco and others. Fully encapsulated these units were
intended for instrumentation, test equipment and other similar
purposes. Both linear and switching supplies are available, if a
switching based unit is used the 5V output must be loaded in order
for the regulator to function properly. I recommend linear based
supplies if available.

Battery
Bias

If you wish to use
batteries for bias, simply delete the pots R1 and R4, then connect the 9V
batteries directly to R3 and R5 respectively. Connect the negative side to
the resistors and positive to ground. Leave C3 and C4 in place. Use one
battery per channel for best results and note that the batteries will
require infrequent replacement, perhaps once every couple of years or so. No
other bias supply is necessary in this case.

Optional
Bias Supply

This will be necessary if the optional filament supply design is used in
place of the brick. See the provided schematic for details. The 7912 series
regulator will provide adequate noise performance as there is additional low
pass filtering between the supply and the grid bias pots.

Optional
Filament Supply

A constant current
filament supply may be used if desired, and is shown in the filament and
bias supply options schematic. Two independent filament supplies are
required, one for each tube. Do not ground the filament supply in the power
supply chassis. Simply delete C6, R6, and R7, connect FILS+ to pin 1 and
FILS—to pin 4 of the respective DHT. Connect pin 4 of each DHT to chassis
ground in the pre-amplifier through a 10Ω
resistor which will also allow you to measure and set the bias more
accurately (and safely.) The values shown in the schematic are intended for
use with the 26, however alternative values are given for operation with the
01. The monolithic regulators should be TO-220 types and must be adequately heatsinked. For adequate performance the voltage across each regulator
should be no less than 3.5V. Dual secondary transformers are readily
available from Allied Signal and other manufacturers.

Using 01's
in place of 26's

Use the optional
filament regulators and substitute the component values listed in the
schematic notes. These will furnish 5V @ 250mA for the filaments, and the
socket connections are as noted above for the 26. In addition change R11 in
the plate supply to approximately 78.7KΩ
to reduce plate voltage to a safe value for the 01. Bias should be set so
that the plate current is approximately 3mA per tube. (Roughly –9V at the
grid.)

Other Construction
Details

I built my prototype on a single chassis, however unless you use a
particularly large chassis I definitely recommend you not follow my lead on
this one. I had particular problems with hum pick up in the left channel
audio transformer which necessitated its relocation. Stray fields from the
power transformer were also picked up by the 26's, a problem only resolved
by installing a shield can over the power transformer.

Layout is relatively non-critical; however, keep the wiring between the
grids, coupling caps and input attenuators/pots as short as possible. Should
high frequency oscillation be noted add a 1KΩ
grid stop resistor right at the socket on each DHT. These were not necessary
in my unit; lead lengths in the prototype including the capacitor body is
under 1.5".

I seriously recommend the use of separate chassis in the construction of
this design. All input connectors should be isolated from chassis and a
ground buss or star grounding scheme should be used with only one
connection to the chassis.

Despite the choice of a
surplus brick for filament and bias supply, the balance of components were
all of high quality.

A conventional volume
pot may be used; however, I really recommend the use of good quality stepped
attenuators if you feel so inclined. I used a pair of ladder type stepped
attenuators I purchased from Michael Percy a few years ago. The solitary
coupling capacitor is a REL TFT type, but paper types may also be employed.

Output transformers
should be mounted close to the individual tube to which it is connected, and
far away from anything that produces a magnetic field. In the likely case
that you are using a transformer other than the specified HA-133 it is
important to assure that the transformers provide some level of magnetic
shielding to reduce induced pickup.

Here is the parts list for the version most like
what I built; it assumes the use of the aforementioned surplus power supply
"brick."

Optional components
shown in the filament and bias supply schematic are not listed here.

Many of the components
required to build this project are available from Digikey, Mouser, Antique
Electronic Supply and others.

This design is copyrighted and remains the property of Kevin Kennedy/KTA
and may be used for noncommercial uses only. For commercial use of any of
the circuits presented here, please contact me (kkennedy@positive-feedback.com)
for licensing details.